The present invention generally relates to a semiconductor integrated circuit, and in particular to a semiconductor circuit which includes a digital circuit and a digital/analog converter and outputs an analog signal.
Conventionally, there is a semiconductor integrated circuit which includes a digital circuit and a digital/analog converter (hereafter referred to as D/A converter). A digital signal derived from the digital circuit is converted to an analog signal by the D/A converter. The converted analog signal is fed to an external circuit through output terminals. A cathode ray tube controller (hereafter referred to as CRTC) is an example of the above semiconductor integrated circuit.
Upon production, the semiconductor integrated circuit must be tested in order to investigate whether it can normally operate without an erroneous operation. The semiconductor integrated circuit is connected to a tester upon testing. The tester provides the semiconductor integrated circuit to be tested with test data and detects whether predetermined output data corresponding to the inputted test data are available at output terminals of the semiconductor integrated circuit. Such a test is called a function test.
However, the conventional semiconductor integrated circuit including the digital circuit and D/A converter has the following disadvantages. Since the semiconductor integrated circuit outputs the analog signal, the test for the built-in digital circuit must be performed on the basis of levels of the analog signal outputted by the semiconductor integrated circuit. This means that the number of levels of the analog signal increases with increasing the number of bits of the digital signal which is supplied to the D/A converter. For this reason, the level detection of the analog signal upon the function test for the digital circuit is very difficult when the number of bits of the digital data to be converted into the analog signal is large.